1. Field of the Invention
The present invention relates to an intersubband transition semiconductor laser, and more particularly, to an intersubband transition semiconductor layer capable of achieving a high output with an economical price, an easy manufacturing process, and a simple structure including a small number of stacked layers.
2. Description of the Related Art
Those skilled in the art have long predicted that a semiconductor superlattice structure may contribute to amplifying an electromagnetic wave to realize a unipolar intersubband transition quantum well semiconductor laser, and have made many efforts to develop the unipolar intersubband transition quantum well semiconductor laser. This type of intersubband transition lasers have the advantages of a tailoring the frequency in the wide-range spectrum, a narrow line width based on the theoretical absence of line-width increasing factors, and low temperature-dependency of an oscillation threshold in comparison to a conventional bipolar semiconductor laser.
A properly designed unipolar intersubband transition quantum well semiconductor laser may emit light having a submillimeter wavelength at a mid/far infrared ray. For example, light with a wavelength ranging from about 3 to 100 μm may be emitted by a carrier transition between quantum confinement states. The wavelength of light emitted may be designed with same heterostructure system over a wide spectrum range. The wavelength band cannot be obtained through a conventional semiconductor laser diode. Also, the unipolar intersubband transition quantum well semiconductor laser can be manufactured on the basis of a sufficiently technically developed III-V compound semiconductor materials (e.g. a heterostructure based on GaAs or InP) which have relatively wide energy bandgaps. For this reason, there is no need to use a material with a small energy bandgap, which is sensitive to temperature and requires complex processes.
Conventional technologies for implementing the unipolar intersubband transition quantum well semiconductor laser include a resonant tunneling structure based on a typical multiple quantum-well structure. For example, W. M. Yee et al. analyzed two kinds of coupled quantum well structures in “Carrier transport and intersubband population inversion in couple quantum well”, Appl. Phys. Lett. 63(8), pp. 1089-1091 (1993). Each of the coupled quantum well structures includes a quantum well for emission that is interposed between energy filter wells, coupled with a quantum well structure interposed between n-type doped injector/collector regions.
In the year of 1994, Faist, Capasso, et al. named a unipolar intersubband transition quantum well semiconductor laser a quantum cascade LASER (QCL), and succeeded in the first emission of light with a wavelength of about 4.2 μm from a GaInAs/AlInAs material-based system. The laser that can be implemented with another material-based system can easily be designed to oscillate at a predetermined wavelength over a wide spectrum range.
The quantum cascade LASER includes an undoped multi-layered semiconductor quantum well structure as an active region. The quantum well active region is separated from a neighboring quantum well active region by an energy relaxation region. For example, a radiative transition between confinement energy states in the quantum well active region may be designed to be a vertical transition occurring in the same quantum well or a diagonal transition occurring between confinement energy states of neighboring quantum wells.
The unipolar laser diode with such a wavelength band may be applied in a variety of fields such as contamination detection, process control, and automotive. Thus, the quantum cascade LASER that can emit mid/far infrared rays has received much commercial and scientific attention.
However, in the conventional quantum cascade LASER, one electron emits N photons, while passing through N unit-cells stacked structures, where each unit cell includes tens of layers and have a quantum well active region and an energy relaxation region. To obtain a sufficient optical output, N must be about 25 to 70. For this reason, the structure is complicated and the manufacturing process is extremely difficult, since a multi-layered structure must be grown epitaxially using equipment such as a molecular beam epitaxy system. Thus, the conventional quantum cascade LASER has been studied and developed to an extremely limited extent.